1. Field of the Invention
The present invention relates to a laser irradiation method and a laser irradiation apparatus to perform the laser irradiation (the laser irradiation apparatus includes a laser oscillator and an optical system to guide a laser beam emitted from the laser oscillator to an object to be irradiated.). In addition, the present invention relates to a method for manufacturing a semiconductor device using the laser irradiation method and the laser irradiation apparatus to perform the laser irradiation. It is noted that the semiconductor device described in this specification includes a general device that can be operated by utilizing the semiconductor characteristic. And thereby a computer, an electro-optical device and the like having the semiconductor device as its component are also included in the semiconductor device.
2. Description of the Related Art
In recent years, research has been extensively conducted on the technology to crystallize an amorphous semiconductor film formed over an insulating substrate such as a glass substrate and to form a semiconductor film having a crystal structure (hereinafter referred to as a crystalline semiconductor film). As the crystallizing method, a thermal annealing method using an annealing furnace, a rapid thermal annealing method (RTA method), a laser annealing method and the like have been examined. It is possible to employ one of these methods or a combination of these methods in crystallization.
The crystalline semiconductor film has a much higher mobility than the amorphous semiconductor film. Therefore, the crystalline semiconductor film is utilized for forming a thin film transistor (TFT), and is further utilized for an active matrix liquid crystal display device and the like with TFT for a pixel portion or both TFT for the pixel portion and TFT for a driver circuit formed over one glass substrate.
Usually, in order to crystallize the amorphous semiconductor film in the annealing furnace, the heating process needs to be performed at a temperature of not less than 600° C. for 10 hours or more. It is quartz that is suitable for a material of a substrate applicable for this crystallization, but a quartz substrate is expensive and is very difficult to be processed into a large substrate. Enlarging the size of the substrate is considered to be one of the means to increase production efficiency, and that is why the research has been done to form a semiconductor over a glass substrate which is inexpensive and which can be easily processed into a large substrate. Recently, it has been examined to use the glass substrate with a side of 1 m or more.
As an example of the research, the thermal crystallization method with metal element disclosed in the published unexamined patent application no. H7-183540 makes it possible to lower the temperature of crystallization that has been regarded as a problem in the conventional method. According to the thermal crystallization method with metal element, the crystalline semiconductor film can be formed by adding a minute amount of nickel, palladium, lead or the like to the amorphous semiconductor film and then heating it at a temperature of 550° C. for four hours. The temperature of 550° C. is lower than the distortion temperature of the glass substrate, and thereby it is not necessary to worry about its deformation and the like.
On the other hand, the laser annealing method makes it possible to give high energy only to the semiconductor film without increasing the temperature of the substrate. Therefore, the laser annealing method is attracting attention because this method can be employed not only to the glass substrate whose distortion temperature is low, but also a plastic substrate and the like.
An example of the laser annealing method is explained as follows. A pulsed laser beam generated from a pulsed laser oscillator, typically an excimer laser, is shaped into square with several centimeters on a side or linear having a length of 100 mm or more on a surface to be irradiated and the laser beam is moved relatively to the object to be irradiated to perform annealing. It is noted that “linear” here does not mean a line strictly but means a rectangle (or an oblong) with a large aspect ratio. For example, linear indicates a rectangle with an aspect ratio of two or more (preferably from 10 to 10000), which is included in a laser beam that is rectangular in shape at the surface to be irradiated (rectangular beam). The laser beam is shaped into linear in order to secure energy density for sufficient annealing to the object to be irradiated though the laser beam may have the rectangular shape or a planar shape when sufficient annealing can be performed to the object to be irradiated.
The crystalline semiconductor film thus manufactured has a plurality of crystal grains assembled and a position and a size of each crystal grain are random. TFT formed over the glass substrate is formed by patterning the crystalline semiconductor film into island shape in order for isolation. In such a case, it was not possible to form the crystal grains as specifying their positions and sizes. Compared to the inside of the crystal grain, the boundary between the crystal grains (crystal grain boundary) has an amorphous structure and an infinite number of recombination centers and trapping centers existing due to crystal defects. It is known that when a carrier is trapped in the trapping center, potential of the crystal grain boundary increases to become a barrier against the carrier, and therefore a current transporting characteristic of the carrier is lowered. Although the crystallinity of the semiconductor film in a channel forming region has a serious influence on characteristics of the TFT, it was almost impossible to form the channel forming region with a single-crystal semiconductor film by eliminating such an influence of the crystal grain boundary.
Recently, attention has been paid to the technique of irradiating continuous wave (CW) laser beam to a semiconductor film while scanning the CW laser beam in one direction to form a single-crystal grain extending long in the scanning direction. A region in which such single-crystal grains are assembled is referred to as a long crystal grain region in this specification. It is considered that it is possible, with this technique, to form a TFT that has almost no crystal grain boundary at least in the channel direction thereof. (for example, US published patent application 2002/0031876 A1)
However, in the crystallizing method using the CW laser oscillator, when a CW YAG laser is used for example, since the CW laser beam having wavelengths absorbed sufficiently in the semiconductor film is utilized, the CW laser beam had to be converted into a harmonic. Therefore, only the laser oscillator that outputs as low as 10 W is utilized, and it is inferior to the excimer laser in terms of productivity. When the CW laser oscillator having a wavelength of 532 nm with an output of 10 W is used to crystallize the semiconductor film having a thickness of about 50 nm, a size of the beam spot has to be made as small as 10−3 mm2 approximately, for example. Here, the output means a power of the laser beam, which is energy per unit time. On the other hand, when an excimer laser is employed, the beam spot can be made as large as 1 cm2. It is noted that the CW laser oscillator with high output, having a wavelength not longer than that of visible light and having a considerably high stability is appropriate in this method. For example, a second harmonic of a YVO4 laser, a second harmonic of a YAG laser, a second harmonic of a YLF laser, a second harmonic of a YAlO3 laser, an Ar laser and the like are applicable. Although the other higher harmonics are applicable for the annealing, they have a disadvantage of small output. It is the object of the present invention to decrease the disadvantage in productivity to a large degree as keeping the advantage of the crystallizing technique using the CW laser oscillator.